Let's back up.
Let's scrap most of the math.
You cut 2 sqs. that were 4&7/8", right?
You cut these on both diagonals to make 8 total triangles, right?
Then you sewed the triangles cut from one sq. to the triangles cut from the other square, right?
And now you're asking what size these sewn together triangles are supposed to be so you can square them up, right?
And you want to know what size the finished block using these HSTs should be, right?
I'd like to know how many HSTs are in the whole block.
What is the name of the block?
If there are a total of 4 HSTs in the block, then they probably should be trimmed to 3.5".....which will give you a block of 6.5".
(The problem with this method is that all your outside edges of the HSTs are on the bias.....a pain in the butt, IMHO.)
I'll be happy to PM with you if you wish.
Jan in VA